This invention relates to projection television systems and specifically to color projection television systems utilizing a single light valve and a device, such as a color wheel, for altering the color of the light emitted by a projection lamp before it impinges on the light valve.
Most commercially available projection video systems utilize separate projection systems for each of the three primary colors. Thus the system require red, green and blue light valves and optical systems which must be accurately converged on the screen. Recently, projection television systems utilizing only a single light valve have been developed. One such system is a color field sequential system, in which the normal video field, 1/60th of a second (16 ms), is broken into three parts, or color fields. These parts are normally equal length, so each color sub-field is 1/180th of a second (5.33 ms). Note that a 1/60 of a second video field is for 60 Hz NTSC systems, 50 Hz non-NTSC system would have a 1/50 of a second (20 ms) video field.
During the three color sub-fields, the light valve is illuminated with red, green and blue light sequentially. While the light valve is illuminated with any given color, the video data corresponding to that color is displayed on the light valve. The eye then fuses the three color sub-fields into a single, full color field. The eye also fuses successive video fields and frames into full motion, full color video. This system requires a device for illuminating the surface of the light valve with the three colors. The simplest of such devices is a rotatable color wheel which serves to change the color of a white projection lamp as it rotates.
Recently, improved light valves particularly suitable for use in projection television systems have become available. One such device is a so-called deformable mirror device (sometimes called a digital mirror device) which is illustrated in U.S. Pat. No. 5,079,544 (the disclosure of which is hereby incorporated by reference as if fully set forth herein) and patents referenced therein, in which the light valve consists of a array of tiny movable mirror-like devices for deflecting a beam of light either to the display screen (on) or away from the display optics (off). By rapidly switching the pixels on and off a grey scale is generated. The pixel array of such a device is quite small in physical size when compared to a comparable liquid crystal display cell. This device requires that all of the light of the projection lamp be focussed on its relatively small surface which can limit the types of projection lamps useable in such a system. However, many otherwise suitable lamps may be deficient in color spectrum. Such lamps may also shift in color temperature as they age. The present system is directed to providing an electronic means for compensating for any color deficiencies in the projection system.
The color wheels for altering the color of the projection lamp are generally manufactured from dichroic filters. These filters suffer from certain drawbacks. The manufacture of dichroic filters is a batch process and there is a sample-to-sample variation in the colorimetry of these filters. Additionally, upon exposure to the intense light of the projection lamp the colors of the dichroic filters will fade. The present device and projection system also provides a means for compensating for batch-to-batch and color shift of dichroic filter wheels. Finally, the device provides a means for adjusting color balance (tint) to user preference.
The present device provides a color control system for projection television displays. The system includes a projection lamp, a light valve and means for varying the color of the light from the projection lamp, typically a color wheel. The color wheel driver provides an output signal which represents the position, that is the color, of the color wheel to a lamp driver. The lamp driver is capable of varying the output power used to drive the lamp in response to the particular color of the color wheel presently in use. For example, if the projection lamp has a deficiency in a certain color the present system provides a larger output power to the projection lamp during the presence of the filter for that color. A user input control provides for changing the color balance of the system based on user preferences and/or any changes occurring in the dichroic filters or projection lamp. A closed loop technique may be used in which a detector senses the emitted color and automatically adjusts color balance.
The disclosure of U.S. Pat. No. 5,079,544 makes reference to the use of a varying neutral density filter to vary the output power of the projection lamp during the course of the addressing of a single pixel. There is no disclosure of the varying of the output power to the projection lamp for color. Thus, U.S. Pat. No. 5,079,544 varies the output of the lamp on the order of microseconds while the present device operates on the order of milliseconds.